To appear in The Blackwell Guide to the Philosophy of Mind Concepts*

The human mind has a prodigious capacity for representation. We aren't limited to thinking about the here and now, just as we aren't limited to thinking about the objects and properties that are relevant to our most immediate needs. Instead we can think about things that are far away in space or time (e.g., Abraham Lincoln, Alpha Centauri) and things that involve considerable abstraction from immediate sensory experience (e.g., democracy, the number pi). We can even think about things that never have or never will exist in the actual world (e.g., Santa Claus, unicorns, and phlogiston). One of the central questions in the history of philosophy has been how we are able to do this. How is it that we are able to represent the world to ourselves in thought? In answering this question, philosophers and psychologists often take our capacity for thought to be grounded in our conceptual abilities. Thoughts are seen as having constituents or parts, namely, concepts.1 As a result, all of science, literature, and the arts—as well as everyday thought—can be seen to stem from the astounding expressive power of the human conceptual system